U.S. Pat. No. A 4 823 946 shows packaging of this type in which two plastic material bottles are envisaged. In the storage position the second bottle is closed by a cap screwed on to its neck. When using and mixing the contents of the two bottles, the cap on the second bottle is unscrewed and the second bottle is placed in the central duct of the intermediate component with a thread able to associate with the thread on the neck of the second bottle receiving the screw cap.
In dual packaging of this type, it is practically imperative for its acceptance by customers, that the insertion of the second bottle into the intermediate component to eject the plug, be carried out by rotation with a limited number of turns and preferably less than one turn.
Since according to U.S. Pat. No. A 4 823 946 the thread on the neck of the second bottle serves both to receive the screw cap during storage, and to associate with the intermediate component, the thread must be of a relatively large pitch so that in use, extensive axial displacement of the neck related to the intermediate component is obtained with a small angle of rotation.
The need for a large pitch thread on the neck of the second bottle effectively imposes the use of plastic material for the second bottle, since with a glass bottle it is practically impossible to produce a thread with a large pitch simultaneously giving a satisfactory seal, owing to the particular manufacturing conditions for such glass bottles.
The two bottle packaging defined above is used for products, some of which keep unsatisfactorily in a plastic material bottle.
This is the case particularly when packaging on the one hand a color solution intended for hair-tinting, and on the other hand, the oxydizer required when using the coloring solution; in order to color the hair with "oxydizing colors", it is necessary to ensure development of the color on the hair by the addition of an oxydizer such as hydrogen peroxide, on the color at the time of its use on the hair. The dual bottle presentation is consequently important owing to the extent of the market for hair coloring materials.
The color solution generally placed in the second bottle, stores poorly inside a polyethylene or polypropylene plastic material bottle, since the oxygen passes through the wall and oxydizes the color before application to the hair.